Saturday, January 5, 2013

Tau Ceti has an enduring whiff of the Sixties about it. Peace, love, dolphins. The connotations are not a fluke: Tau Ceti was lifted out of back-of-the-Greek-alphabet obscurity in 1960, when it and Epsilon Eridani shared the distinction of being the target stars for the Project Ozma SETI experiment.

Yes, that was years before the Summer of Love, but - especially in that pre-Internet era - it took time for ideas to filter into the science fiction popular culture. Before Project Ozma, I doubt that many people had heard of Tau Ceti.

It got the attention of Project Ozma's designers because it is close to Sol (11.9 light years) and similar in spectral type (G8.5). Alpha Centauri is closer and more similar, but the prevailing view at the time was that double (and other multiple) stars went poorly with planets. Indeed this remained a cause for hesitation right up until planets were found in multiple-star systems, recently including Alpha Centauri itself.

And now planets - five of them - have (possibly) turned up around Tau Ceti as well. They have minimum masses between 2 and 6.6 times Earth mass, and orbit between 0.11 and 1.35 AU from the parent star.

Tau Ceti radiates at just over half (0.52) solar luminosity. Thus the fourth planet (at 0.52 AU) and fifth planet orbit respectively at the thermal-balance equivalent of 0.76AU and 1.87 AU from Sol.

A bit farther out than Venus, and considerably farther out than Mars. Neither sounds like Paradise World, but depending on a host of secondary assumptions they might both be in the habitable zone.

Minimum masses are 4.4 and 6.3 Earths, which for the inner planet is all too suggestive of a super Venus. The outer planet could be more promising, and if it has a deep hydrosphere the surface gravity could even be comparable to Earth's.

Proviso time: Sky & Telescope throws some cold water on the whole planetary system. The detection method is experimental, and the indications suggesting planets are no louder than the noise in the data. This does not rule out valid detection, but makes it the equivalent of eavesdropping on an interesting conversation at a loud party.

In this case my gut feeling is that S&T is being a bit too cautious - that the Tau Ceti planets will likely turn out to be real. And along with the Alpha Centauri finding, it seems as though extrasolar planets, including kinda sorta Earthlike ones, are - so to speak - creeping closer to Sol.

Reaching Tau Ceti with any semi-demi-plausible drive is outrageously difficult. Refer to the Alpha Centauri post and comment thread for speculation. But it is perhaps time to assess just where we stand when it comes to planets and life.

According to the Extrasolar Planets Encyclopedia, there are now 854 known, that is to say confirmed, extrasolar planets. (The Tau Ceti system is not listed in their main catalog.) Extrasolar worlds thus outnumber 'major' Solar System planets by a hundred to one, an impressive balance.

Planets are common. Planetary systems are odd. So far as I can tell, we have not yet found any systems that look a lot like ours - rocky inner planets, larger gas or ice-rich outer ones, most or all in near-circular orbits. This may be a mere selection effect.

And in spite of the Kepler findings we have not yet found an 'Earth' - a planet of similar size orbiting at a distance closely equivalent to 1 AU. This could be a different sort of selection effect. We are finding a decent number of kinda-sorta Earths, and hitting bingo depends on the constraints you set.

As for the biological implications? Alas, we still have a data set of precisely one. Which means that we don't really have a clue as to how common life is, or what conditions are needed for it to appear.

Project Ozma, like rocketpunk-era SF, lay in the bright glow of the Miller-Urey experiment in 1952. This experiment showed that, given an atmosphere similar to what was then thought to be Earth's primordial atmosphere, the simpler molecular building blocks of light were easy to produce. Life in a test tube seemed just around the corner.

As with controlled fusion and human-like AI, things have not worked out that way. On the other hand, I don't get the impression that biochemists have made a major research project out of trying to brew up living organisms. The project has no obvious application except to generate media hysteria, not good for funding proposals.

Behind all discussions of life lurks the Fermi Paradox. Yes, it is specific to intelligent (or at least interstellar-signaling) life, not life in general. But the more common life is elsewhere in the universe, the more opportunities it has to make its presence known. And the harder to explain why it hasn't.

Discuss:

The image of dolphins comes from this Flickr page. Back around the 1970s, National Lampoon magazine included dolphins (meaning dolphin intelligence) in a list of Great Disappointments. True? False? Irrelevant?

45 comments:

@RickAnd in spite of the Kepler findings we have not yet found an 'Earth' - a planet of similar size orbiting at a distance closely equivalent to 1 AU. This could be a different sort of selection effect. We are finding a decent number of kinda-sorta Earths, and hitting bingo depends on the constraints you set.

I think Kepler actually isn't capable of identifying a transit of an Earth-sized planet in an Earth-sized orbit around a Sun-like star. The closest it gets are Super-Earths and "MEarths" (Earth-sized planets orbiting M-class stars). And of course, it can only see a fraction of solar systems, those it can see "edge on".

The truth is that our detection methods still favor finding planets close to their stars, and the bigger the planets, the easier they are to find. The good news is that having gas giants in the inner solar system doesn't seem to be the guaranteed non-existence of Earth-style planets that we though it might mean, at least not if Kepler-11 is any indication.

We'll just have to wait for Kepler's successors: a combination of new space telescopes and adaptive optics on the ground.

Behind all discussions of life lurks the Fermi Paradox. Yes, it is specific to intelligent (or at least interstellar-signaling) life, not life in general. But the more common life is elsewhere in the universe, the more opportunities it has to make its presence known. And the harder to explain why it hasn't.

Honestly, the Fermi Paradox has so many possible loopholes that it's not much use. It could just be so difficult from an engineering and energy perspective to create starships that almost nobody does, or they do it so infrequently that it didn't happen in the narrow window of time that we've been observing space with anything other than the naked eye.

Hmm,two topics: Earth-like planets in general that might be close to us and Tau Ceti planets that maybe life-bearing. Both seem to be good possibilities. Tau Ceti planets might be close enough that a probe could be sent to it: said probe could be launched by the end of this century, to arrive on-site before 2200. Such a probe would most likely be the size of a typical attack submarine, with several sub-probes for landing on and orbiting an 'interesting' planets (based on whatever standards of interesting the mission planners use), and a very powerful communications suite.We (humans), have several designs for interstellar probes; I would guess that at the end of this century we will weed out those that are totally impractical. Pity we probably won't live to see it, but I'd sure would like to.

According to Wiki, the signals of Earth-like planets are close enough to the noise level in the instrument to need repeated transits to detect as positive. So it can detect Earth-sized planets, but it definitely isn't easy.

@OrbitingPlutoAccording to Wiki, the signals of Earth-like planets are close enough to the noise level in the instrument to need repeated transits to detect as positive. So it can detect Earth-sized planets, but it definitely isn't easy.

That's better than I thought.

The "at least three observed transits" does put a time constraint on how soon we can confirm them. Even if Kepler spotted an exact Earth-twin, it would take 3 years for it to get at least three transits. More if they need additional transit observations.

I've seen more than a few journals making passing mention that synthetic life is just around the corner. They aren't starting off from scratch in a test tube primordial soup (just add lightning!) but they seem confident they can do it by combining a bunch of genes and proteins all made synthetically.

I've actually got this working theory life is very, very common. I have a feeling that wherever you find liquid H2O you will find life. Certainly on Earth itself, wherever you find liquid water - no matter the possible permutations of pressure, temperature, exogenous energy source - you will find life.

I get the feeling a lot of xenobiologists are thinking the same thing.

The evidence needed, as such, of it will be if we find life at Europa/Enceleadous or even some protected area of Mars.

I would have thought we should focus our efforts on finding life within our solar system. This will increase the possibility of us finding something interesting when we eventually spend the huge resources necessary to send a mission to another star.

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With regards to the Fermi Paradox.

We may comfort ourselves at night with the thought that there might be many benign loopholes to explain the great silence.

I personally think we most definitely shouldn't be broadcasting our location to every star we can see BEFORE we know who we are actually trying to contact.

I think David Brin gave a good warning analogy in one of his blogs. I'm hardly going to travel across the oceans and land in Central Park, Manhattan for the very first time and then shout "Hello, I'm new here, does anyone want to talk!!!! And btw here's my exact home address AND a copy of a few of my children's most treasured music recitals.

With planets or so very common. With life being or so very common on our own planet. I'm begining to worry just a little.

From a rocketpunk fiction point of view. It might make for a great story if we do somehow find out there is some sort of hostile berserker out there and the human race spends the next thousand years trying to hide/relocate and retrieve the Voyager probes.

Locki - I've actually got this working theory life is very, very common.

I agree. Microbial life is probably common. Intelligent life would be very rare by comparison, but with the vast number of planets there probably is a good number of intelligent species in the galaxy.

We may comfort ourselves at night with the thought that there might be many benign loopholes to explain the great silence.

I have never been impressed by the Fermi Paradox. The assumption that an extremely advanced race would check out every single nook and cranny of the galaxy as some sort of manifest destiny is hardly a scientific law. One reason for the great silence could be that no one has bothered with such a silly exploration strategy. Not to mention it might not be technically possible even if theoretically possible.

Another reason for the silence is not there there isn't anybody out there, but that they don't have radios. Human history points to that. Our ancestors 200,000 years ago were about as intelligent as we are today, but we have only been able to produce powerful radio transmissions for about 75 years.

There could be several intelligent species within communications range with highly sophisticated civilizations that never stumbled across what we call technology. Developing science and technology is not destiny.

On the other hand, I don't get the impression that biochemists have made a major research project out of trying to brew up living organismsTo the contrary, this is an active - and exciting - area of research. There are not a lot of groups working in this area, but several articles appear every year investigating various aspects of abiogenesis. Wikipedias article on abiogenesis covers the basics, and has lot of links to actual studies.

Google scholar lists 620 articles on abiogenesis since 2010; slow by the standards of many fields, but still a pretty active field.

As for the implicit "when will scientists make artificial life" the answer is May 20, 2010, when the venter institute generated the first synthetic organism...

...but if you mean when will we replicate an abiogenic process in a lab, the answer is "probably never" - abiogenesis, at least on earth, appears to take hundreds of millions of years.

@LockiThe evidence needed, as such, of it will be if we find life at Europa/Enceleadous or even some protected area of Mars.

All good spots, although I think Mars and Enceladus will be much easier to investigate than Europa. Enceladus has areas where we know that water is being pumped out through the ice (the water volcanoes), so it might be possible to drop a probe through there. Mars could have water near the surface, if we can find it.

Europa, on the other hand, might have its ocean under at least several kilometers of ice. Considering how much difficulty we're having with that mission to drill through two miles of ice in Antarctica to find life in Lake Vostok, you can imagine how hard it would be to set-up a probe to do that on Europa . . . provided there aren't "thin" areas in the ice (which there might very well be - we need to send an orbiter there to do more intensive scanning of the surface).

I personally think we most definitely shouldn't be broadcasting our location to every star we can see BEFORE we know who we are actually trying to contact.

Unless an alien civilization just happens to be in the extremely narrow width of area where they could pick up military and Arecibo telescope radar, we probably don't have to worry about that. The signals become almost impossible to tell from background noise in a few light-years anyways.

@RonThere could be several intelligent species within communications range with highly sophisticated civilizations that never stumbled across what we call technology. Developing science and technology is not destiny.

While I think any technologically advanced civilization would eventually stumble across radio technology, I wonder if you could have a civilization that never uses it for anything more than short-range transmissions. Maybe they would go straight from telegraphs, to wired communications, to fibre optics and satellite lasers, etc.

Another possible explanation for the Fermi Paradox (probably not a very original one at that) is the aliens have discovered that for the same investment in time and energy they can create "baby" or basement universes that can be tailored for whatever they consider to be ideal conditions.

Why expend a fraction of your home star's energy output to go someplace which is probably not suitable anyway when you can inhabit a designer universe where stars, planets and any other factor you can tweak can be comfortable and convenient?

@Bryan"As for the implicit "when will scientists make artificial life" the answer is May 20, 2010, when the venter institute generated the first synthetic organism..."

First synthetic genome, I would say. Of course eventually all the proteins and other non-DNA molecules were replaced by copies produced by the genes on the new genome, but they did start with a preexisting cell with all the necessities apart from DNA.

First synthetic genome, I would say. Of course eventually all the proteins and other non-DNA molecules were replaced by copies produced by the genes on the new genome, but they did start with a preexisting cell with all the necessities apart from DNA.

1. Thanks zmil. That was the exact example I was unsuccessfully trying to dredge out of my holiday brain.

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2. Fermi paradox. I hope I’m not derailing the thread but it’s a natural progression when discussing life.

The sheer numbers of planets of every variety we are finding around virtually every visible star has completely debunked one of my favourite solutions to the “paradox”.

Planets must be the norm around stars. A planetless star system (even binary or trinary) must be a rarity.

There must be billions of earth like (either deep sea vent or surface liquid water with atmosphere) moons and planets out there. Even a moon like Europa is possibly quite earth like in its similarity to our deep sea vents.

I agree there are still plenty of non-sinister solutions. The most likely being

(i) Intelligence rare (after all its occurred for a narrow 4000 year window in our “garden planet’s” 4 billion year lifespan

(ii) Technology forks and variations

(iii) Why would a culture bother anyway ….

(iv) A hard technology limit on the universe making interstellar colonisation/communication practically impossible

Still considering this is an unknown, unknown I would have thought it more prudent not to go shouting at every star in blind ignorance.

There is a plausible, though low probability, of catastrophe of some sort explaining the great silence.

3. Exploration of Europa

I worry about the proposals to look for life on Europa. As someone earlier pointed out we’ve had a hell of a time drilling down through Lake Vostok.

If there were some sort of deep sea Atlantean culture beneath the ice of Europa dropping a nuclear powered, non shielded, hot thermal probe with enough plutonium to melt through 6km of ice is probably not the friendliest way to make first contact.

Even if there were only microbes (most probable scenario) we have to ask ourselves if we really want to drop a radioactive probe into this untouched space wilderness.

3. Liquid H2O and life

I'm sure we’ll one day determine that where conditions allow for a relatively stable environment of liquid H2O and a constant, steady energy source (a given if the H2O is liquid I guess) for a certain minimum period of time (I’m guessing 1 billion years) then self replicating life becomes an inevitability as a result of the stability of that system.

Looking at life on Earth, I see that while there was life for at least 3.5 billion years, for the vast majority of the time the most complex life forms resembled pond scum.

About 500 million years ago, multi cellular life appeared (possibly twice, depending on how you interpret the Ecardian fossils). The reasons for the Cambrian explosion are not clear, and it may be that these conditions have not been created on other, Earth like worlds out there (meaning most planets we would ever encounter would be filled with the equivalent of bacteria or pond scum).

One idea I find compelling is that the act of engulfing mitochondria in a symbiotic relationship is the factor which powered the evolution of advanced life on Earth, and that it is possible that nothing like this has happened "out there", meaning higher life forms are simply not possible since the cellular machinery cannot generate enough energy.

I have heard of the theory that while life is common in the universe, that multi-cellular life is much more rare, and that intelligent life is vanishingly rare- perhaps even there only being one or two intelligent species per galaxy existing at any one time. Maybe that explains the Fermi Paradox; there might be 100 billion intelligent species out there, but at one or two per galaxy, we may never run across another one. We would have better luck creating our own "aliens".

If the heat to melt down through the ice comes from Pu238 rather than some sort of fission reactor, it will be quite safe for any lifeform it encounters. Pu238 is used for radioisotope thermal generators *because* it emits only alpha particles which get stopped in less than a mm, & does not emit penetrating radiation like gamma rays. Eating PU238 or breathing in Pu238 dust would be unhealthy, but sitting nearby or touching a chunk of Pu238 would have the same hazard as touching the element on a stove.

I’ll state this as an exclusion clause since this is all purely wild speculation. We have a sample size of 1 afterall and it depends on whether you are philosophically inclined to believe in the anthropic principle.For whatever reason I seem to be inclined to think we “aren’t that special”.

Thucydides said...

One idea I find compelling is that the act of engulfing mitochondria in a symbiotic relationship is the factor which powered the evolution of advanced life on Earth, and that it is possible that nothing like this has happened "out there", meaning higher life forms are simply not possible since the cellular machinery cannot generate enough energy.

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Mitochondria are fascinating (a separate genome to us eukaryotes with more in common with the prokaryotes) but I’m inclined to think the development of eukaryotes was pretty much inevitable once life started. Its the only way to deal with the deadly byproduct of early photosynthetic microscopic life – oxygen! George Lucas has a lot to explain about making mitochondria mainstream in the sci-fi geek crowd (our whole biochemistry year was groaning/crying at that midichlorian line in Phantom Menace).

I think the rate limiting step that best explains the Fermi paradox is the unlikely development of intelligence. In all probability over the 1 billion year of history of multicellular life -intelligence has only developed once. 2.5 million years ago hominids first appeared.

Homo Sapiens have probably been around only 1-200,000 years. BUT, the real explosion occurred only 60,000 years ago when we suddenly developed the tools to explode out of Africa and settle every corner of the world within the space of a few thousand years. Interestingly this coincides with a time period when we very nearly became extinct (about 2000 humans left).

One theory I’ve seen crop up a few times was that this near extinction event drastically selected for genes that allowed us to survive during harsh drought. From what I’ve seen they think the gene was for language.

You’ll also note mitochondrial Eve (the one female we can all trace ourselves back to) existed about 60,000 years ago.

I think the event that makes a technical civilisation possibly was the extreme selection pressures that almost made homo sapiens extinct 60,000 years ago. There was just the right amount of selection pressure for evolution to take a chance on a hugely expensive brain that also gave us the ability to form a technical civilisation.

Without that near extinction event we’d still be running round like hunter gatherers like our hominid forebears.

It’s a very finely balanced thing. With just 2000 humans remaining at that time period (60,000 years ago) if a sabre tooth tiger had of stumbled across the wrong family at the wrong time we probably wouldn’t have had either the genes necessary for intelligence – or the genetic diversity to survive. I’m inclined to think that if there were more humans surviving at that period in time (say 100,000) evolution would have never taken the chance on a huge brain that used up a lot of scares energy and simultaneously made 50% of our population far less mobile (wide hips on female) AND made childbirth deadly (huge head to squeeze out of hips designed for bipedal motion)

"Yes, that was years before the Summer of Love, but - especially in that pre-Internet era - it took time for ideas to filter into the science fiction popular culture. Before Project Ozma, I doubt that many people had heard of Tau Ceti."

Well...

Just off the top of my head, Tau Ceti turns up in Heinlein and Dickson in the 50s (Time for the Stars and Dorsai!, respectively). Given the tendency for Golden Age space opear to use fictitious names for stars far, far from Sol, or just not name stars at all, I doubt Tau Ceti was any less prominent than any real stars that might have been mentioned in SF literature.

Some (pretty typical) misunderstandings here. All the Paradox is is a framework for discussion. It doesn't presume anything has to be true, or anything else has to be false. It was deigned as an anwer to what was becoming a pretty basic assumption at the time. It simply asks: if intelligence is common, and if intelligence would explore, and if the galaxy is explorable, why don't we see any evidence of it?

No answer or opinion can possibly debunk or devalue the Paradox, because it is inherently designed to elicit such anwers and opinions.

Locki:That's a pretty good theory. Intelligence requiring some tromatic trigger event to spark it. I still think that even if intelligences is common, technological civilizations may only be a small subset. There might be a 1,000 intelligent species in our galaxy alone, but we might be the only technilogical one. Or it could be because of a completely different reason; I guess we'll just have to continue to explore the universe to find out.

I just read an article about habital moons around gas giant worlds. I'm thinking that above a certain size, (probably a large percent of Earth's mass), that worlds differenciate into core, mantle, and crust. Details like atmosphere and hydrosphere (oceans) would be determined by the exact composition of the accretion disk they formed out of. I'm thinking that the composition of planets are more determined by the starting mixture and the position of the planet-forming-disk in the paticular star-system. The rate that we're finding exoplanets and the advances we're making in detecting smaller planets, it's no wonder that scientests are so confedent that we'll find an Earth-like planet in the habitable zone in the very near future.

Fermi's paradox as such does not specify any conclusion. But I think it's a fairly natural usage to use 'Fermi's paradox' to refer to some of the possible implications.

I'm not sure I read DORSAI (I know I read some of his work), but I certainly read TIME FOR THE STARS. Tau Ceti must have flown right past me, as it were.

My sheer guess - worth what you paid - is that life is common, but not ubiquitous. There could be many cases, perhaps most, where it chokes itself off almost at the outset: A bloom, then a die-off, with not enough variations appearing quickly enough for the engine to keep turning over.

If life does take hold, there may then be a succession of unlikelihoods to reach starfaring (or even star-communicating) civilizations. Complexity ... intelligence ... self-awareness ... technology?

Even if dolphins are really, really smart, that little issue with hands puts a real crimp in their travel plans.

"Even if dolphins are really, really smart, that little issue with hands puts a real crimp in their travel plans"

“For instance, on the planet Earth, man had always assumed that he was more intelligent than dolphins because he had achieved so much—the wheel, New York, wars and so on—whilst all the dolphins had ever done was muck about in the water having a good time. But conversely, the dolphins had always believed that they were far more intelligent than man—for precisely the same reasons.”

Douglas Adams later revealed the Dolphins left the Earth by their own means, after leaving a final message for us......

How likely would an Earth-sized moon be around a gas giant? The Galileans are cool, but they're also pretty small in terms of mass (they combined represent less than 10% of the mass of Earth). They're also quite icy - if they migrated into the inner solar system, they'd become water balls before gradually losing all their water.

I wonder if an Earth-like moon would have to form in the inner solar system, after the gas giant migrated inward. As I mentioned above, it's possible to have a gas giant and earth-ish sized planets in the inner solar system together if Kepler-11 is any indication. Maybe the gas giant's gravity would slam material together to form captured moons, some of them quite large . . . assuming its gravity doesn't mess with their formation (I've heard that Mars is as small as it is because of Jupiter).

They might be more likely around Neptune-sized gas giants. Heck, a super-Earth of two Earth-masses and a Neptune-mass planet (17 earth-masses) would have a mass ratio of 8.5. That's more or less a double planet system right there.

1775Brett, a Jupiter-sized or super-Jupiter planet having a moon half to three quarters Earth-sized isn't too far fetched; this moon would only be 1 to .5% of the parent planet's mass. Moreover, astronomers think that not all gas giants form in the outer system and migrate inward, but that some form in the inner system. Also, some large moons could benefit from tidal heating. Just because our system doesn't have Earth or Mars sized moons, doesn't mean that another system won't. We've discovered stranger things in distant star systems.

The problem with Habitable gas giant moons is that we now know they are sitting in the way of every piece of junk that their primaries can suck in. Imagine how often such worlds get in the way of a chunk of asteroid or comet, a la Shoemaker-Levy 9...

The gas giants probably suck in most of that stuff themselves. As long as the moon isn't "hugging" the gas giant in a tight orbit outside the Roche Limit, that could really help in minimizing the impacts.

The mass of the gas giant matters, too. Jupiter takes a bunch of impacts just because it's so massive, especially compared to the rest of the planets in the solar system.

More problematic is radiation. God knows what a Jupiter-sized planet's Van Allen belts would be like if it was getting hit with far stronger solar wind due to being in the habitable zone instead of the icy outer solar system, although an Earth-sized moon might have its own magnetic fields to mitigate some of that.

It doesn't matter that the primary absorbs most of the impacts. The figure of merit is the frequency of dangerous objects entering the system, which the primary greatly increases. The more jusnk coming through the satellite orbits, the higher likelihood that a satellite will get hit.

Maybe we should check the impact craters on Io, Europa, Ganymede, and Calisto before we come to any conclusions. Incidentally, Calisto actually has a lower level of radiation than Earth, due to its orbit. The other Galliaen moons have much higher radiation levels.

"Maybe we should check the impact craters on Io, Europa, Ganymede, and Calisto before we come to any conclusions."

Io isn't relevant, because it has a surface that is highly dynamic on a very short timescale, thanks to hyper-vulcanism. Same-same Europa, due to it's icy crust. Cratering in general on Callisto and Ganymede might be as recent as 1 billion years. Much more recent events, and their frequency, would require much more detailed data collection and anlysis.

Tony said:"Io isn't relevant, because it has a surface that is highly dynamic on a very short timescale, thanks to hyper-vulcanism. Same-same Europa, due to it's icy crust. Cratering in general on Callisto and Ganymede might be as recent as 1 billion years. Much more recent events, and their frequency, would require much more detailed data collection and anlysis."

If there are craters visable on Io or Europa, then the frequency of impacts are greater than the rate of surface remodeling, which will tell us something useful about the rate of such strikes. Impact craters on Callisto and Ganymede will show us a detailed pattern of impacts over a significant period of time. Any scientific long term outpost (manned or robotic), we put on those moons would need that kind of information. I would think that any probe of the Jupiter system would make such observations as part of its routine; I'd think that any landers and/or rovers on those moons would also take some sort of measurements to gather data on any impact craters they encountered. But I guess we'll just have to wait, because sending a probe to Jupiter isn't exactly a trivial matter; years and billions of dollers for any that make it off the launch pad. Anyway, it's fun to speculate.

One resource a civilization based on a Jovian moon (or its analogue around a distant star) is the flux tube that Io (or it's equivalent) produces.

Huge amounts of high amperage/high voltage electrical energy would be a boon to the colony, and smaller electrodynamic tethers free flying through the magnetosphere could be tailored to produce energy at whatever amount and rate desired.

Habitable moons of gas giants are a cool concept - which makes the bombardment issue a serious bummer!

The specific points made in both directions are above my pay grade to fully resolve, but at a gut-feeling level, it does seem that giant planets would make the space around them a (relative) shooting gallery.

But to further complicate things, I just recently read (but neglected to bookmark) an argument that most planet-bearing stars have on order of 10 times as much circumstellar dust as Sol does.

Which could suggest that most planetary systems are shooting galleries. Or at least the sorts of planetary systems we readily detect.

A dinosaur killer every million years is a problem for a planet developing native intelligent life, but anyone that can get to that planet from another solar system will have the technology to divert anything on a collision course & so could safely colonize such a planet.

The colonists might decide to emulate one of the saner decisions of the Moties in the backstory to _The Mote in Gods Eye_ and move the larger asteroids into orbits like L4 & L5 points that can't eventually impact the habitable planet.

@RickBut to further complicate things, I just recently read (but neglected to bookmark) an argument that most planet-bearing stars have on order of 10 times as much circumstellar dust as Sol does.

Which could suggest that most planetary systems are shooting galleries. Or at least the sorts of planetary systems we readily detect.

It could be, as you said, an artifact of how we detect planets - like how we find so many Hot Jupiters.

That said, I think it really depends on how long the interval between Chixculub-level impacts and down actually is. One every ten million years might not retard land life that much. In the ten million years that was the Paleocene, we went from small tiny land mammals to Titanoides. Sea life, of course, would likely be even less affected and thus highly complex and diverse on such a world.

In fact, that might push a species into greater intelligence, sort of like how the Toba Eruption and collapse of the rainforests in Africa at the advent of the Pleistocene pushed out ancestors out into a new environment.

If it's a lot more common that that, and some of the impacts are worse than anything we've seen (such as a 20-30 kilometer impactor) . . . then yeah, that's bad news. You could still have an oxygen-bearing biosphere, but I suspect that life would seem rather less "complex" compared to our own. Any fast breeding life that could come back after calamity would dominate that world.

Another candidate for explaining the Fermi Paradox.

The specific points made in both directions are above my pay grade to fully resolve, but at a gut-feeling level, it does seem that giant planets would make the space around them a (relative) shooting gallery.

That's why I think they'd be more common around smaller gas giants, where it would almost be like a double-planet system. Even a Saturn-sized gas giant would probably draw a lot fewer impactors.

@ThucydidesHuge amounts of high amperage/high voltage electrical energy would be a boon to the colony, and smaller electrodynamic tethers free flying through the magnetosphere could be tailored to produce energy at whatever amount and rate desired.

If you don't mind living in the bath of radiation that it would entail.

We are assuming that Human beings are fully capable of inhabiting a space environment, so they are either equipped with massively shielded habitats.

Alternatively, they use the energy to create protected zones around their colonies, or if we postulate real handwavium technology, post humans or their robotic replacements feed off the energy directly.

Living inside a hollowed out asteroid is slightly past PMF, for the other ideas YMMV.

And for all we know catastrophe might be a very good explanation, a strong filter we've missed by definition. Frequency of dinosaur-killers is a minor issue, you want to look at the frequency of ecosphere-killers. Venus runaways, Snowball Earths, close-range supernovae or gamma ray bursters, Venusian surface remodelings, *big* impactors.

Earth has avoided any -- as far as we know, though I'm not sure we would know if life got re-set from scratch partway through the Archaean -- but that doesn't mean we're normal.

Of course, it's possible that industrial civilizations don't live long. Or a bit less depressingly, that interstellar travel is really hard and no one does it. That leads to "there are lots of intelligent species but we will *never ever meet them*."

That's my inclination, as well. I think interstellar travel requires a trade-off between building functional ships that can stay functional after centuries or millenia in transit, or expending vast amounts of energy to accelerate them to faster speeds (hopefully figuring out a way to slow them down at the destination).

Both seem like pretty huge barriers to interstellar travel, so many civilizations just might not bother with it. God knows our own solar system is pretty vast - that's a ton of real estate to play around with already.

On what Locki said (tongue in cheek) about less mobile, less efficient big brained humans evolving without the help the glacial period of Marine Isotopic 6 to bottle neck the population. I think it already happened they were called Neanderthals. Big Brains and intelligence doesn't necessarily mean a species will develop the cooperative skills to create complex civilizations. Interestingly enough the Lunar cycle may have been useful to those early ancestors of ours since it allowed them to track periods of low tide during which they could gather protein and calorie rich marine resources.

Homo sapiens could best be describe as the "Goldilocks Hominin" too much evolutionary pressure one way or another either produces "robust hominins" that are built to hunt and forage more effectively but require to much caloric intake to spend there time on developing complex culture like the Neanderthals( not that they didn't develop culture, it simply wasn't as complex and didn't develop as rapidly as in sapiens sapiensvs. sapiens neanderthalensis). Or evolution takes the other route and you end up with "gracile hominins" with small but efficient brains and bodies but lacking the environmental drivers and selective pressures to develop the necessary neurological and physiological features to create complex culture.

It's one of the reasons among others I think our ancestors had a semi-aquatic stage at some point. If hairlessness and fattiness had originally developed as part of a Plains Ape Theory we would be more robust and have kinky hair as a dominant feature like Neanderthals may have. All though there is some debate over whether kinky hair was a Denisovan trait and Papua New Guineans got their blonde genes from Neanderthals instead of getting both from there Neanderthal makeup, although most of the research suggests at least some Neanderthals had kinky hair and Sapiens have always been straight hair dominant and acquired kinky hair through cross speciation or developed it later as a Nilotic adaptation. Forgive me if any of that has fallen on racially sensitive ears it was not meant to offend, anyhow I find the whole Australoid hypothesis kind of racially offensive in a way since it was touted and sort of supports Pro-meditteranean/Semitic racial superiorists like Garibaldi as well as an Indo-European bias with an Out of India hypothesis.